The present invention is concerned with gripper bars which are used in a machine for converting plate-shaped matter or sheets into packages and enables the pulling and positioning of the sheets as they are carried through various sequence of printing, cutting, embossing, creasing and/or waste-stripping stations.
Gripper bars are generally fitted on two lateral chains at regular distances and extend crosswise to the travelling direction of the plate-shaped sheets. Inside the machine, the chains having the bars follow a circuit initially composed by successive horizontal translational movement from a starting position along a horizontal path through the various processing stations and then, at the end of the path, move in an upward arc to a return path which then terminates in a downward curved arc to the starting position for the path extending through the stations. When the bar is in the starting position, it is aligned with stops of a feeder table. At this stage, the grippers are opened by a control device which, depending on the structure of the machine, can be either an integral part of the gripper bar or a separate element of the machine.
When a subsequent piece of a sheet of material is pushed onto the feeder table toward the front stops with the leading edge being moved between the lower gripper counterparts and the upper pressing fingers of the open grippers, the gripper opening mechanism will then be actuated in a reverse direction in such a way that the new plate-shaped piece will be gripped in order to be carried to the first processing station in the course of the first translational movement of the gripper bar along the path between the stations. In view of the fact that in every processing station the plate-shaped pieces are to be stopped in order to allow the accomplishment of the corresponding cutting, waste-stripping and similar operations, the pieces are to be transferred from one station to the other as commanded by the successive translation composed of a combination of an acceleration, a deceleration, and either a dwell or a standstill.
Research, which was undertaken with the purpose of increasing in the production speeds of the machines, has shown the necessity to have the above-mentioned successive translations be accomplished with increasingly higher speeds in an increasingly shorter time rate of the operating cycle. So, for instance, it is possible to reach a throughput and production speed of more that 9,000 pieces per hour with a cycle duration of about 0.4 seconds, of which about 0.3 seconds are absorbed alone by the translation of the plate-shaped sheet through the station with a length of about 1 meter. Since the bars are connected only at their ends, the bending and torsional stresses applied to the bars by the acceleration and deceleration is readily evident.
Furthermore, these gripper bars should have sufficient sturdiness to prevent any deformation when the gripper opening axle, which may be part of the bar, is driven in a rotary direction contrary to the numerous springs which guarantee the closing of the grippers. On account of the small size of the grippers fitted along the trailing edge of the bar, the rear springs ensure their closing can only be shorter and stronger enabling them thereby to ensure the necessary gripping action for picking up the plate-shaped sheet of material. Consequently, a very strong opening torque is applied to the axle, which actuates the upper pressing fingers. Moreover, as the grippers are permanently fitted on the opening axle, this axle must be made from steel with appropriately large dimensions.
When taking into consideration the above-mentioned stresses, the bar is presently made from a more or less rectangular tube of an appropriate size. Similarly, the bar fixtures on the chains are to have adequate dimensions. Such a gripper bar is described, for instance, in French Patent 11 78 295. The bars of the French Patent have the drawback because their weight acts on the machine production speed. In fact, the movable assembly comprising the gripper bars, the pertaining fixtures and the two lateral chains on which they are fitted represent a mass of high inertia momentum of which any acceleration are contained between limits set by the resistance of gripper bar drive elements to the stresses resulting from the acceleration and, likewise, by the power of the main motor.
In order to enable the reduction of the gripper bar height, French Patent 88 37 80 proposes to have the full rectangular profile bar machined with recesses to allow a fitting of the elastic pivoting gripper on the leading edge of the bar in such a way as to prevent the gripper in the rest position from exceeding the upper and lower sides of the bar. However, such recesses or slots, which are made perpendicular to the lengthwise direction of the bar, greatly weaken the resistance of the bar to bending and torsion.
With this design, the height of these bars determine actually the minimum vertical shift to be effected by the lower movable platen when the plate-shaped sheets are being carried from one station to the other by these bars. In order to reduce the shifting range to a minimum with a view to increase the production speed, it is appropriate to have the movable platen accomplish the shortest shift possible. Consequently, the height of the gripper bar has to be reduced to a minimum. for example to the height of the gripper bar chains. Moreover, as the grippers are presently made entirely of metal, the clanking noise originating from their opening and closing action causes a considerable noise level which should be eliminated, if possible. Finally, such bars are generally connected at both ends to two laterally movable drive chains by means of so-called "floating" fixtures so that the plate-shaped material can be accurately positioned and aligned on the feeder table with regard to the travelling direction and to the processing station. Such "floating" devices are described, for example, in DE-C 25 20 231 and generally include a connection device engaged at one end of the gripper bar and elastically attached to the corresponding lateral drive chains. However, this connection has been conceived in such a way that every drive chain will have to be moved apart crosswise in order to disengage the bar end of the corresponding connecting device. Such "floating " fixtures of the prior art have, thus, the drawback of needing the dismantling of the drive chain from the machine, for instance in cases where a gripper bar would have to be exchanged. Such fixtures are generally of heavy weight, of difficult execution and are, thus, expensive. In addition, as disclosed in GB-A 10 03 838, the "floating" fixture includes a connection device fitted on the bar on a special link of the drive chain. The connection device is provided with two arms held in place on the outer surface of the lower and upper walls of the bar, as well as with two male and female knuckles; one close to the leading edge and the other to the trailing edge of the bar. The special link is provided with a male knuckle. The male and female knuckles are connected to one another by means of two rods extending across corresponding holes; the male knuckle being fitted on the rod so as to enable a slight shift with an elastic catch-up in the travelling direction of the drive chains. A relevant fact is that with this connection device, the height of the two arms is to be added to the one of the bar.